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Date: Wed, 12 May 2021 09:58:01 +0800
From: Ming Lei <ming.lei@...hat.com>
To: John Garry <john.garry@...wei.com>
Cc: axboe@...nel.dk, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
kashyap.desai@...adcom.com, chenxiang66@...ilicon.com,
yama@...hat.com, dgilbert@...erlog.com
Subject: Re: [PATCH v2] blk-mq: Use request queue-wide tags for tagset-wide
sbitmap
On Wed, May 12, 2021 at 12:15:43AM +0800, John Garry wrote:
> The tags used for an IO scheduler are currently per hctx.
>
> As such, when q->nr_hw_queues grows, so does the request queue total IO
> scheduler tag depth.
>
> This may cause problems for SCSI MQ HBAs whose total driver depth is
> fixed.
>
> Ming and Yanhui report higher CPU usage and lower throughput in scenarios
> where the fixed total driver tag depth is appreciably lower than the total
> scheduler tag depth:
> https://lore.kernel.org/linux-block/440dfcfc-1a2c-bd98-1161-cec4d78c6dfc@huawei.com/T/#mc0d6d4f95275a2743d1c8c3e4dc9ff6c9aa3a76b
>
> In that scenario, since the scheduler tag is got first, much contention
> is introduced since a driver tag may not be available after we have got
> the sched tag.
>
> Improve this scenario by introducing request queue-wide tags for when
> a tagset-wide sbitmap is used. The static sched requests are still
> allocated per hctx, as requests are initialised per hctx, as in
> blk_mq_init_request(..., hctx_idx, ...) ->
> set->ops->init_request(.., hctx_idx, ...).
>
> For simplicity of resizing the request queue sbitmap when updating the
> request queue depth, just init at the max possible size, so we don't need
> to deal with the possibly with swapping out a new sbitmap for old if
> we need to grow.
>
> Signed-off-by: John Garry <john.garry@...wei.com>
> ---
>
> Please retest, thanks! For some reason I could not recreate the original
> issue, but I am using qemu...
>
> Changes since v1:
> - Embed sbitmaps in request_queue struct
> - Relocate IO sched functions to blk-mq-sched.c
> - Fix error path code
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 42a365b1b9c0..9a012e0818cb 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -507,11 +507,9 @@ static void blk_mq_sched_free_tags(struct blk_mq_tag_set *set,
> struct blk_mq_hw_ctx *hctx,
> unsigned int hctx_idx)
> {
> - unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
> -
> if (hctx->sched_tags) {
> blk_mq_free_rqs(set, hctx->sched_tags, hctx_idx);
> - blk_mq_free_rq_map(hctx->sched_tags, flags);
> + blk_mq_free_rq_map(hctx->sched_tags, set->flags);
> hctx->sched_tags = NULL;
> }
> }
> @@ -521,12 +519,10 @@ static int blk_mq_sched_alloc_tags(struct request_queue *q,
> unsigned int hctx_idx)
> {
> struct blk_mq_tag_set *set = q->tag_set;
> - /* Clear HCTX_SHARED so tags are init'ed */
> - unsigned int flags = set->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
> int ret;
>
> hctx->sched_tags = blk_mq_alloc_rq_map(set, hctx_idx, q->nr_requests,
> - set->reserved_tags, flags);
> + set->reserved_tags, set->flags);
> if (!hctx->sched_tags)
> return -ENOMEM;
>
> @@ -544,16 +540,40 @@ static void blk_mq_sched_tags_teardown(struct request_queue *q)
> int i;
>
> queue_for_each_hw_ctx(q, hctx, i) {
> - /* Clear HCTX_SHARED so tags are freed */
> - unsigned int flags = hctx->flags & ~BLK_MQ_F_TAG_HCTX_SHARED;
> -
> if (hctx->sched_tags) {
> - blk_mq_free_rq_map(hctx->sched_tags, flags);
> + blk_mq_free_rq_map(hctx->sched_tags, hctx->flags);
> hctx->sched_tags = NULL;
> }
> }
> }
>
> +static int blk_mq_init_sched_shared_sbitmap(struct request_queue *queue)
> +{
> + struct blk_mq_tag_set *set = queue->tag_set;
> + int ret;
> +
> + /*
> + * Set initial depth at max so that we don't need to reallocate for
> + * updating nr_requests.
> + */
> + ret = blk_mq_init_bitmaps(&queue->sched_bitmap_tags,
> + &queue->sched_breserved_tags,
> + set, MAX_SCHED_RQ, set->reserved_tags);
> + if (ret)
> + return ret;
> +
> + sbitmap_queue_resize(&queue->sched_bitmap_tags,
> + queue->nr_requests - set->reserved_tags);
> +
> + return 0;
> +}
> +
> +static void blk_mq_exit_sched_shared_sbitmap(struct request_queue *queue)
> +{
> + sbitmap_queue_free(&queue->sched_bitmap_tags);
> + sbitmap_queue_free(&queue->sched_breserved_tags);
> +}
> +
> int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
> {
> struct blk_mq_hw_ctx *hctx;
> @@ -578,12 +598,25 @@ int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
> queue_for_each_hw_ctx(q, hctx, i) {
> ret = blk_mq_sched_alloc_tags(q, hctx, i);
> if (ret)
> - goto err;
> + goto err_free_tags;
> + }
> +
> + if (blk_mq_is_sbitmap_shared(q->tag_set->flags)) {
> + ret = blk_mq_init_sched_shared_sbitmap(q);
> + if (ret)
> + goto err_free_tags;
> +
> + queue_for_each_hw_ctx(q, hctx, i) {
> + hctx->sched_tags->bitmap_tags =
> + &q->sched_bitmap_tags;
> + hctx->sched_tags->breserved_tags =
> + &q->sched_breserved_tags;
> + }
The above assignment can be folded into blk_mq_init_sched_shared_sbitmap().
> }
>
> ret = e->ops.init_sched(q, e);
> if (ret)
> - goto err;
> + goto err_free_sbitmap;
>
> blk_mq_debugfs_register_sched(q);
>
> @@ -603,7 +636,10 @@ int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
>
> return 0;
>
> -err:
> +err_free_sbitmap:
> + if (blk_mq_is_sbitmap_shared(q->tag_set->flags))
> + blk_mq_exit_sched_shared_sbitmap(q);
> +err_free_tags:
> blk_mq_sched_free_requests(q);
> blk_mq_sched_tags_teardown(q);
> q->elevator = NULL;
> @@ -641,5 +677,7 @@ void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
> if (e->type->ops.exit_sched)
> e->type->ops.exit_sched(e);
> blk_mq_sched_tags_teardown(q);
> + if (blk_mq_is_sbitmap_shared(q->tag_set->flags))
> + blk_mq_exit_sched_shared_sbitmap(q);
The above two lines can be moved to blk_mq_sched_tags_teardown().
> q->elevator = NULL;
> }
> diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
> index 5b18ab915c65..aff037cfd8e7 100644
> --- a/block/blk-mq-sched.h
> +++ b/block/blk-mq-sched.h
> @@ -5,6 +5,8 @@
> #include "blk-mq.h"
> #include "blk-mq-tag.h"
>
> +#define MAX_SCHED_RQ (16 * BLKDEV_MAX_RQ)
> +
> void blk_mq_sched_assign_ioc(struct request *rq);
>
> bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 2a37731e8244..e3ab8631be22 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -13,6 +13,7 @@
> #include <linux/delay.h>
> #include "blk.h"
> #include "blk-mq.h"
> +#include "blk-mq-sched.h"
> #include "blk-mq-tag.h"
>
> /*
> @@ -466,19 +467,39 @@ static int blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
> return -ENOMEM;
> }
>
> -int blk_mq_init_shared_sbitmap(struct blk_mq_tag_set *set, unsigned int flags)
> +int blk_mq_init_bitmaps(struct sbitmap_queue *bitmap_tags,
> + struct sbitmap_queue *breserved_tags,
> + struct blk_mq_tag_set *set,
The 'set' parameter can be killed, meantime pass 'node' & 'alloc_policy',
just like blk_mq_init_bitmap_tags()'s type, then blk_mq_init_bitmaps()
can be re-used by blk_mq_init_bitmap_tags() for avoiding to duplicate
bitmap allocation code.
> + unsigned int queue_depth, unsigned int reserved)
> {
> - unsigned int depth = set->queue_depth - set->reserved_tags;
> + unsigned int depth = queue_depth - reserved;
> int alloc_policy = BLK_MQ_FLAG_TO_ALLOC_POLICY(set->flags);
> bool round_robin = alloc_policy == BLK_TAG_ALLOC_RR;
> - int i, node = set->numa_node;
>
> - if (bt_alloc(&set->__bitmap_tags, depth, round_robin, node))
> + if (bt_alloc(bitmap_tags, depth, round_robin, set->numa_node))
> return -ENOMEM;
> - if (bt_alloc(&set->__breserved_tags, set->reserved_tags,
> - round_robin, node))
> + if (bt_alloc(breserved_tags, set->reserved_tags,
s/set->reserved_tags/reserved/
Thanks,
Ming
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